a. Field of Invention
The present invention relates generally to improvements to the delivery of drugs, particularly to systems for subcutaneous injection/aspiration. More specifically this invention provides a method and device to the identification of specific tissue types (or soft-tissue density types) based on using a pressure measurement.
b. Description of the Prior Art
Infusion pumps devices and systems are well known in the medical arts, for use in delivery or dispensing a prescribed medication to a patient. The administration of prescribed drugs has been described in the literature as administration to a patient through infusion tubing and an associated catheter or the like, thereby introducing the drug intravenously. These systems are usual capable of determining infusion line occlusion. Line occlusions cause the pressure in the syringe to increase. Systems in the prior art have been developed to identify a predetermined threshold or to monitor pressure to determine means for selecting ranges of occlusion pressures to insure patient safety. U.S. Pat. Nos. 5,295,967; 4,731,058; and 5,080,653 show systems (with syringe pumps or the like) which are adequate for the intended use of intravenous drug delivery and more specifically for monitoring occlusion during infusion. However, these systems do not provide a means for drug delivery or aspiration subcutaneously via a hypodermic needle.
Accurately positioning a hollow-bore needle within tissues to delivery medication within tissue structures has long been a challenge in both medicine and dentistry. The inability to accurately position a hollow-bore needle within specific tissues (i.e. soft-tissues) or organs can lead to a failed medical objective. Locating pathologic tissue types (i.e. neoplasia, tumors, cysts and the like) is critical to aspiration of these tissues as well as the infusion of therapeutic medications to treat these local lesions of the body. Hence locating a specific anatomically site has been previously assisted with the use of ionizing radiation, ultrasound, MRI, electrical-stimulators and other invasive diagnostic devices that require secondary techniques to be employed to assist the practitioner to determining the accuracy of the placement of a needle within tissue.
Pain, tissue damage and post-op complications have long been tolerated as negative side effects from the use of existing hypodermic drug delivery injection systems. This is well documented in both the dental and medical literature. The pain and tissue damage are a direct result of uncontrolled flow rate in conjunction with excessive pressures created during the administration of drug solutions within the tissue spaces. Subjective pain response of a patient has been demonstrated to be minimized at specific flow rates during the administration of a drug. Also, it has been scientifically demonstrated that particular pressures (excessive without occlusion, per se) for a specific tissue type will cause damage. However, the present inventor has discovered that the amount of pain felt by a patient can be minimized with the use of a specific flow rate range in conjunction with a specified exit pressure range during the delivery of fluids (drugs). Moreover when drugs are delivered at this prescribed low ranges of pressure and fluid flow, tissue damage is minimized as well. It is also necessary that this system have the capability to aspirate under controlled conditions of rate and pressure to avoid the same negative side effects during fluid movement. U.S. Pat. No. 5,180,371 to Spinello, incorporated herein by reference, presented an invention, which allowed a rate to be set for the drug via a hypodermic needle. That invention however did not disclose means of determining, detecting or monitoring pressure during the administration of a drug. U.S. Pat. No. 6,113,574 to Spinello discloses an injection device in which a pressure sensor switch is used to determine during a PDL injection whether the liquid from a needle is injected into the proper location, or is leaking out into a patient's mouth or to some other location. However, the patent does not address the problem of identifying the tissue in which an injection is being made.
During the early 1980's, several researchers ( See for instance Rood, The Pressure Created by Inferior Alveolar Injections, British Dental J. 144:280-282 (1978); Walton and Abbot, Periodontal Ligament Injection; a Clinical Evaluation JADA. (October 1981); Smith and Walton, Periodontal Ligament Injection; Distribution of Injected Solution Oral Surg 55:232-238 (1983)) clearly demonstrated and concluded that the pressure created by the injected fluid is critical to preventing tissue damage and a pain response. Variability, different collagen types and connective tissue densities result in different tissue compliance and distensibility. These variations are found between subjects and within the individual subjects. Rood in his 1978 article states that “[t]he relationship between rate of injection and pressure rise seen clearly with the smaller volumes was lost when 2.0 ml was injected. Several high pressures were recorded and some unexpected low ones. Many tracings showed a pattern suggestive of tissue disruption and it is possible that said low pressures were due to the fluid no longer being contained within the pterygomandible space as the volume injected was similar to the previously estimated volume of the tissue space.” Hence, it appears that the rate of flow is not directly related to pressure during an interstitial injection.
Smith and Walton described in their article identified supra discussed above that they have performed a histologic animal study (canines) using a technique to calibrate manual pressures produced. They concluded that the “Volume injected and needle location were not always related to distribution. Injecting under moderate to strong back pressure gave deeper and more widespread dye penetration.” This once again confirms that pressure is the critical variable in the distribution of the solution within tissues and the volume is not always related to the pressure produced.
Pashley, Nelson & Pashley in “Pressures Created by Dental Injections” (J Dent Res 1981) used a pressure transducer and fixed flow rate created by a motor driven traditional syringe clearly demonstrated that different tissues have different tissue compliance. Interstitial pressure variability was statistically and clinically significant even with a fixed flow rate. Therefore, it may be concluded that they produced great variations of pressure by using a metered flow rate.
Pertot and Dejou described in their article “Effects of the force developed during periodontal ligament injections in dogs” (Oral Surg. Oral Med, Oral Pathol. 1992) how they used a syringe coupled to a miniature force transducer and found a positive correlation between the number of osteoclasts and the force applied on the syringe plunger, which indicated the pressure generated in the PDL space enhanced osteoclastic activity. This experiment again indicates that pressure is a critical factor to tissue damage and is dependent on the resistance encountered and not the flow rate of the solution into the tissues.
Prior art references are known which attempt to utilize a pressure transducer to measure the pressure within the syringe (See for instance U.S. Pat. No. 5,295,967). A major deficiency of these systems is their inability to adjust the flow rate and/or pressure of the fluid to compensate for changes in resistances throughout the system, or to the exit pressure. (Exit pressure refers to the fluid pressure just downstream of the needle tip within the patient's body). Moreover, the prior art references fail to provide any means of determining this exit pressure.
U.S. Pat. No. 6,200,218, a parent of the present application and incorporated herein by reference, discloses an automatic injection device that includes a drive mechanism that causes a therapeutic fluid to flow from a cartridge supported by a cartridge holder, a tube and a handle with an injection needle. The drive mechanism is connected to an electric motor and a sensor positioned at the motor output that measures the force applied by the motor to the drive mechanism. This force is then used to determine an internal characteristic such as a force or internal pressure generated during the injection process. This characteristic is then used as a control parameter by a microprocessor or controller which generates corresponding commands to the drive mechanism. In a particularly advantageous embodiment, the characteristic is used to calculate an exit pressure at which fluid ejected by the device through an elongated tube. The electric motor, is then operated in such a manner that the exit pressure is maintained at a predetermined level to insure that a patient does not suffer pain and/or tissue damage.